Engine exhaust purification device

ABSTRACT

A microprocessor ( 6 ) computes an oxygen storage amount of a catalyst ( 3 ) separately for a high speed component and a low speed component in accordance with real characteristic. A target air-fuel ratio of an engine ( 1 ) is computed and air-fuel ratio control of the engine ( 1 ) it is performed so that the high speed component is constant. The deterioration of the catalyst ( 3 ) is determined by integrating the high speed component of the oxygen storage amount computed during the air-fuel ratio control process for a predetermined number of times, and comparing its average value with a determining value. The oxygen storage amount of the high speed component which is sensitive to catalyst deterioration is integrated so as to obtain a highly precise determining result to determine the deterioration.

FIELD OF THE INVENTION

[0001] The present invention relates to an engine exhaust purificationdevice provided with a catalyst.

BACKGROUND OF THE INVENTION

[0002] JP-A-H9-228873 published by the Japanese Patent Office in 1997discloses a technique wherein an oxygen amount stored in a three-waycatalyst (hereafter, “oxygen storage amount”) is estimated based on anengine intake air amount and an air fuel ratio of an exhaust flowinginto the catalyst, and engine air-fuel ratio control is performed sothat the oxygen storage amount of the catalyst is constant.

[0003] To maintain the NOx (nitrogen oxides), CO and HC (hydrocarbon)conversion efficiency of the three-way catalyst at a maximum, thecatalyst atmosphere must be maintained at the stoichiometric air-fuelratio. If the oxygen storage amount of the catalyst is maintainedconstant, oxygen in the exhaust is stored in the catalyst even if theair-fuel ratio of the exhaust flowing into the catalyst temporarilybecomes lean, and conversely, oxygen stored in the catalyst is releasedeven if the air-fuel ratio of the exhaust flowing into the catalysttemporarily becomes rich, so the catalyst atmosphere can be maintainedat the stoichiometric air-fuel ratio.

[0004] Therefore, in an exhaust purification device performing this typeof control, it is required to calculate the oxygen storage amountprecisely to maintain the conversion efficiency of the catalyst at ahigh level, and various methods of computing the oxygen storage amounthave been proposed.

SUMMARY OF THE INVENTION

[0005] However, as the maximum oxygen storage amount slightly decreasesdue to catalyst deterioration, the target amount undergoes a relativeshift from a suitable value and the conversion efficiency of thecatalyst decreases, i.e., there is a risk that the exhaust performancewill fall with time. To determine the catalyst deterioration, oxygensensors may be installed upstream and downstream of the catalyst, andthe deterioration determined by comparing the number of times theiroutputs invert, or alternatively, the difference of the maximum valueand minimum value of the output each time the output of the downstreamoxygen sensor invert during a predetermined number of times that theoutput of the upstream oxygen sensor inverts, may be computed, anddeterioration determined when its average value is greater than areference value.

[0006] In the above air-fuel ratio control using the three-way catalyst,if an A/F sensor (linear oxygen sensor) having linear characteristicaccording to the air-fuel ratio upstream is provided upstream of thecatalyst to precisely determine the oxygen storage amount, as theamplitude of the output of the A/F sensor is small, and the number ofinversions of the downstream oxygen sensor decreases the more stablyair-fuel ratio control is performed, the catalyst deteriorationdetermining frequency is smaller in the above prior art, and in theworst case, deterioration may not be determined at all.

[0007] It is therefore an object of this invention, which was conceivedin view of the above problems, to provide an exhaust purification devicepermitting determination of catalyst deterioration without depending onthe output inversion of the exhaust oxygen sensor.

[0008] In order to achieve above object, this invention provides anengine exhaust purification device comprises a catalyst provided in anengine exhaust passage, a sensor which detects an exhaust characteristicflowing into the catalyst, and a microprocessor programmed to compute anoxygen storage amount of the catalyst using the detected exhaustcharacteristic, to compute a target air-fuel ratio of the engine basedon the computed oxygen storage amount such that the oxygen storageamount of the catalyst is a predetermined target value, and to determinea deterioration of the catalyst based on an integrated value of theoxygen storage amount for a predetermined time.

[0009] Further this invention provides an engine exhaust purificationdevice comprises, a catalyst provided in an engine exhaust passage, afirst sensor which detects an exhaust characteristic of flowing into thecatalyst, a second sensor which detects an air-fuel ratio of exhaustflowing out of the catalyst, and a microprocessor programmed to computethe oxygen storage amount of the catalyst using the detected exhaustcharacteristic, to perform reset processing which initializes the oxygenstorage amount to a maximum value when the air-fuel ratio of the exhaustfrom the catalyst detected via the second sensor exceeds a leandetermining value, and initializes the oxygen storage amount to aminimum value when the air-fuel ratio of the exhaust from the catalystdetected via the second sensor exceeds a rich determining value, tocompute a target air-fuel ratio of an engine so that the oxygen storageamount of the catalyst is a predetermined target value based on thecomputed oxygen storage amount, and to compare a reset processingfrequency with a determining value, and determine that the catalyst hasdeteriorated when the reset processing frequency exceeds the determiningvalue.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a schematic diagram of an exhaust purification deviceaccording to this invention.

[0011]FIG. 2 is a diagram showing the oxygen release characteristics ofa catalyst.

[0012]FIG. 3 is a flowchart showing a routine for computing an oxygenstorage amount of the catalyst.

[0013]FIG. 4 is a flowchart showing a subroutine for computing an oxygenexcess/deficiency amount in exhaust flowing into the catalyst.

[0014]FIG. 5 is a flowchart showing a subroutine for computing an oxygenrelease rate of a high speed component.

[0015]FIG. 6 is a flowchart showing a subroutine for computing the highspeed component of the oxygen storage amount.

[0016]FIG. 7 is a flowchart showing a subroutine for computing a lowspeed component of the oxygen storage amount.

[0017]FIG. 8 is a flowchart showing a routine for determining a resetcondition.

[0018]FIG. 9 is a flowchart showing a routine for performing reset ofthe computed oxygen storage amount.

[0019]FIG. 10 is a flowchart showing a routine for computing a targetair fuel ratio based on the oxygen storage amount.

[0020]FIG. 11 is a diagram showing how a rear oxygen sensor output andhigh speed component vary when the oxygen storage amount is controlledto be constant.

[0021]FIG. 12 is a flowchart showing the details of a processing routineaccording to a first embodiment related to catalyst deteriorationdetermination.

[0022]FIG. 13 is a flowchart showing how processing is performedaccording to the aforesaid first embodiment.

[0023]FIG. 14 is a flowchart similar to FIG. 12, but showing the detailsof a processing routine according to a second embodiment related tocatalyst deterioration determination.

[0024]FIG. 15 is a diagram similar to FIG. 13, but showing howprocessing is performed according to the aforesaid second embodiment.

[0025]FIG. 16 is a flowchart similar to FIG. 12, but showing the detailsof a processing routine according to a third embodiment related tocatalyst deterioration determination.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Referring to FIG. 1 of the drawings, an exhaust passage 2 of anengine 1 is provided with a catalyst 3, front wide range air-fuel ratiosensor 4 (hereafter referred to as front A/F sensor), a rear oxygensensor 5 and controller 6.

[0027] A throttle 8, and an air flow meter 9 which detects the intakeair amount adjusted by the throttle 8, are provided in an intake passage7 of the engine 1. In addition, a crank angle sensor 12 which detectsthe engine rotation speed of the engine 1 is provided.

[0028] The catalyst 3 is a catalyst having a three-way catalystfunction. The catalyst 3 purifies NOx, HC and CO with maximum efficiencywhen the catalyst atmosphere is at the stoichiometric air-fuel ratio.The catalyst carrier of the catalyst 3 is coated with an oxygen storagematerial such as cerium oxide, and the catalyst 3 has the function ofstoring or releasing oxygen according to the air-fuel ratio of theinflowing exhaust (referred to hereafter as oxygen storage function).

[0029] Here, the oxygen storage amount of the catalyst 3 may bepartitioned into a high speed component HO2 which is stored and releasedby a noble metal in the catalyst 3 (Pt, Rh, Pd), and a low speedcomponent LO2 which is stored and released by the oxygen storagematerial in the catalyst 3. The low speed component LO2 represents thestorage and release of a larger amount of oxygen than the high speedcomponent HO2, but its storage/release rate is slower than that of thehigh speed component HO2.

[0030] Further, this high speed component HO2 and low speed componentLO2 have characteristics as follows:

[0031] When oxygen is stored, oxygen is stored preferentially as thehigh speed component HO2, and begins to be stored as the low speedcomponent LO2 when the high speed component HO2 has reached a maximumcapacity HO2MAX and can no longer be stored.

[0032] When oxygen is released, and the ratio of the low speed componentLO2 to the high speed component HO2 (LO2/HO2) is less than apredetermined value, i.e. when the high speed component is relativelylarge, oxygen is preferentially released from the high speed componentHO2, and when the ratio of the low speed component LO2 to the high speedcomponent HO2 is larger than the predetermined value, oxygen is releasedfrom both the high speed component HO2 and low speed component LO2 sothat the ratio of the low speed component LO2 to the high speedcomponent HO2 does not vary.

[0033]FIG. 2 shows the experimental results for these characteristics.The vertical axis shows released amount from the high speed componentHO2, and the horizontal axis shows released amount from the low speedcomponent LO2. If three different amounts are experimentally releasedfrom effectively the same release start points (X₁, X₂, X₃), the releasefinish points are X₁′, X₂′, X₃′ and the ratio of low speed component tohigh speed component is constant when release is complete.

[0034] As a result, it appears that when oxygen release begins, theoxygen is released from the high speed component so that the high speedcomponent decreases, and when the ratio of low speed component to highspeed component reaches a predetermined ratio, this ratio issubsequently maintained, i.e., oxygen is released while moving on astraight line L shown in the Figure. Here, the low speed component isfrom 5 to 15, and preferably approximately 10, relative to the highspeed component 1. The same characteristics are obtained even when therelease start point lies in the region below the line L.

[0035] When the release start point is in the region to the left of theline L (Y in the Figure), oxygen is released effectively along thestraight line connecting the start point and finish point Y′.

[0036] Returning to FIG. 1, the front A/F sensor 4 provided upstream ofthe catalyst 3 outputs a voltage according to the air-fuel ratio of theexhaust flowing into the catalyst 3. The rear oxygen sensor 5 provideddownstream of the catalyst 3 detects whether the exhaust air-fuel ratiodownstream of the catalyst 3 is rich or lean with the stoichiometricair-fuel ratio as a threshold value. Here, an economical oxygen sensorwas provided downstream of the catalyst 3, but an A/F sensor which candetect the air fuel ratio continuously can be provided instead.

[0037] The cooling water temperature sensor 10 which detects thetemperature of the cooling water is fitted to the engine 1. The detectedcooling water temperature is used for determining the running state ofthe engine 1, and also for estimating the catalyst temperature of thecatalyst 3.

[0038] The controller 6 comprises a microprocessor, RAM, ROM and I/Ointerface, and it computes the oxygen storage amount of the catalyst 3(high speed component HO2 and low speed component LO2) based on theoutput of the air flow meter 9, front A/F sensor 4 and cooling watertemperature sensor 10.

[0039] When the high speed component HO2 of the computed oxygen storageamount is greater than a predetermined amount (e.g., half the maximumcapacity HO2MAX of the high speed component), the controller 6 makes theair fuel ratio of the engine 1 rich, makes the air-fuel ratio of theexhaust flowing into the catalyst 3 rich, and decreases the high speedcomponent HO2. Conversely, when it is less than the predeterminedamount, the controller 6 makes the air fuel ratio of the engine 1 lean,makes the air-fuel ratio of the exhaust flowing into the catalyst 3lean, increases the high speed component HO2, and maintains the highspeed component HO2 of the oxygen storage amount constant.

[0040] A discrepancy may arise between the computed oxygen storageamount and real oxygen storage amount due to computational error, so thecontroller 6 resets the computational value of the oxygen storage amountwith a predetermined timing based on the air-fuel ratio of the exhaustdownstream of the catalyst 3, and corrects this discrepancy from thereal oxygen storage amount.

[0041] Specifically, when it is determined that the air-fuel ratiodownstream of the catalyst 3 is lean based on the output of the rearoxygen sensor 5, it is determined -that at least the high speedcomponent HO2 is maximum, and the high speed component HO2 is reset tomaximum capacity. When it is determined by the rear oxygen sensor 5 thatthe air fuel ratio downstream of the catalyst 3 is rich, oxygen is nolonger being released not only from the high speed component HO2 butalso from the low speed component LO2, so the high speed component HO2and high speed component LO2 are reset to minimum capacity.

[0042] Next, the control performed by the controller 6 will bedescribed.

[0043] First, the computation of the oxygen storage amount will bedescribed, followed by resetting of the computational value of theoxygen storage amount, and air-fuel ratio control of the engine 1 basedon the oxygen storage amount.

[0044] According to the routine as shown by FIG. 3, first, in a step S1,the outputs of the cooling water temperature sensor 10, crank anglesensor 12 and air flow meter 9 are read as running parameters of theengine 1. In a step S2, a temperature TCAT of the catalyst 3 isestimated based on these parameters. In a step S3, by comparing theestimated catalyst temperature TCAT and a catalyst activationtemperature TACTo (e.g. 300° C.), it is determined whether or not thecatalyst 3 has activated.

[0045] When it is determined that the catalyst activation temperatureTACTo has been reached, the routine proceeds to a step S4 to compute theoxygen storage amount of the catalyst 3. When it is determined that thecatalyst activation temperature TACTo has not been reached, processingis terminated assuming that the catalyst 3 does not store or releaseoxygen.

[0046] In the step S4, a subroutine (FIG. 4) for computing an oxygenexcess/deficiency amount 02IN is performed, and the oxygenexcess/deficiency amount of the exhaust flowing into the catalyst 3 iscomputed. In a step S5, a subroutine (FIG. 5) for computing an oxygenrelease rate A of the high speed component of the oxygen storage amountis performed, and the oxygen release rate A of the high speed componentis computed.

[0047] Further, in a step S6, a subroutine (FIG. 6) for computing thehigh speed component HO2 of the oxygen storage amount is performed, andthe high speed component HO2 and an oxygen amount OVERFLOW overflowinginto the low speed component LO2 without being stored as the high speedcomponent HO2, are computed based on the oxygen excess/deficiency amount02IN and the oxygen release rate A of the high speed-component.

[0048] In a step S7, it is determined whether or not all of the oxygenexcess/deficiency amount 02IN flowing into the catalyst 3 has beenstored as the high speed component HO2 based on the overflow oxygenamount OVERFLOW. When all of the oxygen excess/deficiency amount 02IVhas been stored as the high speed component (OVERFLOW=0), processing isterminated. In other cases, the routine proceeds to a step S8, asubroutine (FIG. 7) is performed for computing the low speed componentLO2, and the low speed component LO2 is computed based on the overflowoxygen amount OVERFLOW overflowing from the high speed component HO2.

[0049] Here, the catalyst temperature TCAT is estimated from the coolingwater temperature of the engine 1, the engine load and the enginerotation speed, but a temperature sensor 11 may also be attached to thecatalyst 3 as shown in FIG. 1 and the temperature of the catalyst 3measured directly.

[0050] When the catalyst temperature TCAT is less than the activationtemperature TACTo, the oxygen storage amount is not computed, but thestep S3 may be eliminated, and the effect of the catalyst temperatureTCAT may be reflected in the oxygen release rate A of the high speedcomponent or an oxygen storage/release rate B of the low speedcomponent, described later.

[0051] Next, a subroutine performed from steps S4 to S6 and in the stepS8 will be described.

[0052]FIG. 4 shows the subroutine for computing the oxygenexcess/deficiency amount 02lN of the exhaust flowing into the catalyst3. In this subroutine, the oxygen excess/deficiency amount 02IN of theexhaust flowing into the catalyst 3 is computed based on the air-fuelratio of the exhaust upstream of the catalyst 3 and the intake airamount of the engine 1.

[0053] First, in a step S11, the output of the front A/F sensor 4 andthe output of the air flow meter 9 are read.

[0054] Next, in a step S12, the output of the front A/F sensor 4 isconverted to an excess/deficiency oxygen concentration FO2 of theexhaust flowing into the catalyst 3 using a predetermined conversiontable. Here, the excess/deficiency oxygen concentration FO2 is arelative concentration based on the oxygen concentration at thestoichiometric air-fuel ratio. If the exhaust air-fuel ratio is equal tothe stoichiometric air-fuel ratio, it is zero, if it is richer than thestoichiometric air-fuel ratio it is negative, and if it is leaner thanthe stoichiometric air-fuel ratio, it is positive.

[0055] In a step S13, the output of the air flow meter 9 is converted toan intake air amount Q using a predetermined conversion table, and in astep S14, the intake air amount Q is multiplied by the excess/deficiencyoxygen concentration FO2 to compute the excess/deficiency oxygen amount02IN of the exhaust flowing into the catalyst 3.

[0056] As the excess/deficiency oxygen concentration FO2 has the abovecharacteristics, the excess/deficiency oxygen amount O2IN is zero whenthe exhaust flowing into the catalyst 3 is at the stoichiometricair-fuel ratio, a negative value when it is rich, and a positive valuewhen it is lean.

[0057]FIG. 5 shows a subroutine for computing the oxygen release rate Aof the high speed component of the oxygen storage amount. In thissubroutine, as the oxygen release rate of the high speed component HO2is affected by the low speed component LO2, the oxygen release rate A ofthe high speed component is computed according to the low speedcomponent LO2.

[0058] First, in a step S21, it is determined whether or not a ratioLO2/HO2 of low speed component relative to the high speed component isless than a predetermined value AR (e.g. AR=10). When it is determinedthat the ratio LO2/HO2 is less than the predetermined value AR, i.e.,when the high speed component HO2 is relatively larger than the lowspeed component LO2, the routine proceeds to a step S22, and the oxygenrelease rate A of the high speed component is set to 1.0 expressing thefact that oxygen is released first from the high speed component HO2.

[0059] On the other hand, when it is determined that the ratio LO2/HO2is not less than the predetermined value AR, oxygen is released from thehigh speed component HO2 and the low speed component LO2 so that theratio of the low speed component LO2 to the high speed component HO2does not vary. The routine then proceeds to a step S23, and a value ofthe oxygen release rate A of the high speed component is computed whichdoes not cause the ratio LO2/HO2 to vary.

[0060]FIG. 6 shows a subroutine for computing the high speed componentHO2 of the oxygen storage amount. In this subroutine, the high speedcomponent HO2 is computed based on the oxygen excess/deficiency amount02IN of the exhaust flowing into the catalyst 3 and the oxygen releaserate A of the high speed component.

[0061] First, it is determined in a step S31 whether or not the highspeed component HO2 is being stored or released based on the oxygenexcess/deficiency amount O2IN.

[0062] When the air-fuel ratio of the exhaust flowing into the catalyst3 is lean and the oxygen excess/deficiency amount 02IN is larger thanzero, it is determined that the high speed component HO2 is beingstored, the routine proceeds to a step S32, and the high speed componentHO2 is computed from the following equation (1):

HO2=HO2z+O2IN  (1)

[0063] where: HO2z: value of high speed component HO2 on immediatelypreceding occasion.

[0064] On the other hand, when it is determined that the oxygenexcess/deficiency amount O2IN is less than zero and the high speedcomponent is being released, the routine proceeds to a step S33, and thehigh speed component HO2 is computed from the following equation (2):

HO2=HO2z−O2IN×A  (2)

[0065] where: A: oxygen release rate of high speed component HO2

[0066] In steps S34, S35, it is determined whether or not the computedHO2 exceeds the maximum capacity HO2MAX of the high speed component, orwhether it is not less than a minimum capacity HO2MIN (=0).

[0067] When the high speed component HO2 is greater than the maximumcapacity HO2MAX, the routine proceeds to a step S36, the overflow oxygenamount (excess amount) OVERFLOW flowing out without being stored as thehigh speed component HO2 is computed from the following equation (3):

OVERFLOW=HO2−HO2MAX  (3),

[0068] and the high speed component HO2 is limited to the maximumcapacity HO2MAX.

[0069] When the high speed component HO2 is less than the minimumcapacity HO2MIN, the routine proceeds to a step S37, the overflow oxygenamount (deficiency amount) OVERFLOW which was not stored as the highspeed component HO2 is computed by the following equation (4):

OVERFLOW=HO2−HO2MIN  (4),

[0070] and the high speed component HO2 is limited to the minimumcapacity HO2MIN. Here, zero is given as the minimum capacity HO2MIN, sothe oxygen amount which is deficient when all the high speed componentHO2 has been released is computed as a negative overflow oxygen amount.

[0071] When the high speed component HO2 lies between the maximumcapacity HO2MAX and minimum capacity HO2MIN, the oxygenexcess/deficiency amount 02IN of the exhaust flowing into the catalyst 3is all stored as the high speed component HO2, and zero is set to theoverflow oxygen amount OVERFLOW.

[0072] Here, when the high speed component HO2 is greater than themaximum capacity HO2MAX or less than the minimum capacity HO2MIN, theoverflow oxygen amount OVERFLOW which has overflowed from the high speedcomponent HO2 is stored as the low speed component LO2.

[0073]FIG. 7 shows a subroutine for computing the low speed componentLO2 of the oxygen storage amount. In this subroutine, the low speedcomponent LO2 is computed based on the overflow oxygen amount OVERFLOWwhich has overflowed from the high speed component HO2.

[0074] According to this, in a step S41, the low speed component LO2 iscomputed by the following equation (5):

LO2=LO2z+OVERFLOW×B  (5)

[0075] where: LO2z: immediately preceding value of low speed componentLO2, and

[0076] B: oxygen storage/release rate of low speed component.

[0077] Here, the oxygen storage/release rate B of the low speedcomponent is set to a positive value less than 1, but actually hasdifferent characteristics for storage and release. Further, the realstorage/release rate is affected by the catalyst temperature TCAT andthe low speed component LO2, so the storage rate and release rate can beset to vary independently. In this case, when the overflow oxygen amountOVERFLOW is positive, oxygen is in excess, and the oxygen storage rateat this time is set to for example a value which is larger the higherthe catalyst temperature TCAT or the smaller the low speed componentLO2. Also, when the overflow oxygen amount OVERFLOW is negative, oxygenis deficient, and the oxygen release rate at this time may for examplebe set to a value which is larger the higher the catalyst temperatureTCAT or the larger the low speed component LO2.

[0078] In steps S42, S43, in the same way as when the high speedcomponent HO2 is computed, it is determined whether or not the computedlow speed component LO2 has exceeded a maximum capacity LO2MAX or isless than a minimum capacity LO2MIN (=0).

[0079] When maximum capacity LO2MAX is exceeded, the routine proceeds toa step S44, an oxygen excess/deficiency amount 02OUT which hasoverflowed from the low speed component LO2 is computed from thefollowing equation (6):

LO2OUT=LO2−LO2MAX  (6)

[0080] and the low speed component LO2 is limited to the maximumcapacity LO2MAX. The oxygen excess/deficiency amount O2OUT flows outdownstream of the catalyst 3.

[0081] When the low speed component LO2 is less than the minimumcapacity, the routine proceeds to a step S45, and the low speedcomponent LO2 is limited to the minimum capacity LO2MIN.

[0082] Next, the resetting of the computed value of the oxygen storageamount performed by the controller 6 will be described. By resetting thecomputed value of the oxygen storage amount under predeterminedconditions, computational errors which have accumulated so far areeliminated, and the computational precision of the oxygen storage amountcan be improved.

[0083]FIG. 8 shows the details of a routine for determining the resetcondition. This routine determines whether or not a condition forresetting the oxygen storage amount (high speed component HO2 and lowspeed component LO2) holds from the exhaust air-fuel ratio downstream ofthe catalyst 3, and sets a flag Frich and a flag Flean.

[0084] First, in a step S51, the output of the rear oxygen sensor 5which detects the exhaust air-fuel ratio downstream of the catalyst 3 isread. Subsequently, in a step S52, the rear oxygen sensor output R02 iscompared with a lean determining threshold LDT, and in a step S53, therear oxygen sensor output R02 is compared with the rich determiningthreshold RDT.

[0085] As a result of these comparisons, when the rear oxygen sensoroutput RO2 is less than the lean determining threshold LDT, the routineproceeds to a step S54, and the flag Flean is set to “1” showing thatthe lean reset condition for the oxygen storage amount holds. When therear oxygen sensor output RO2 exceeds the rich determining thresholdRDT, the routine proceeds to a step 555, and the flag Frich is set to“1” showing that the rich reset condition for the oxygen storage amountholds.

[0086] When the rear oxygen sensor output RO2 lies between the leandetermining threshold LDT and rich determining threshold RDT, theroutine proceeds to a step S56, and the flags Flean and Frich are set to“0” showing that the lean reset condition and rich reset condition donot hold.

[0087]FIG. 9 shows a routine for resetting the oxygen storage amount.

[0088] According to this, in steps S61, S62, it is determined whether ornot the lean reset conditions or rich reset conditions hold based on thevariation of the values of the flags Flean and Frick

[0089] When the flag Flean changes from “0” to “1”, and it is determinedthat lean reset conditions hold, the routine proceeds to a step S63, andthe high speed component HO2 of the oxygen storage amount is reset tothe maximum capacity HO2MAX. At this time, resetting of the low speedcomponent LO2 is not performed. On the other hand, when the flag Frichchanges from “0” to “1”, and it is determined that rich reset conditionshold, the routine proceeds to a step S64, and the high speed componentHO2 and low speed component LO2 of the oxygen storage amount arerespectively reset to the minimum capacities HO2MIN, LO2MIN.

[0090] The reason why resetting is performed under these conditions isthat as the oxygen storage rate of the low speed component LO2 is slow,oxygen overflows downstream of the catalyst even if the low speedcomponent LO2 has not reached maximum capacity when the high speedcomponent HO2 reaches maximum capacity, and when the exhaust air-fuelratio downstream of the catalyst becomes lean, it may be considered thatat least the high speed component HO2 has reached maximum capacity.

[0091] When the exhaust air fuel ratio downstream of the catalystbecomes rich, oxygen is not released from the low speed component LO2which is released slowly. Therefore, it may be considered that the highspeed component HO2 and low speed component LO2 are both not beingstored and are at minimum capacity.

[0092] Next, the air-fuel ratio control performed by the controller 6(oxygen storage amount constant control) will be described.,

[0093]FIG. 10 shows a routine for computing a target air fuel ratiobased on the oxygen storage amount.

[0094] According to this, in a step S71, the high speed component HO2 ofthe present oxygen storage amount is read. In a step S72, a deviationDHO2 (=oxygen excess/deficiency amount required by catalyst 3) betweenthe current high speed component HO2 and a target value TGHO2 of thehigh speed component, is computed. The target value TGHO2 of the highspeed component is set to, for example, half of the maximum capacityHO2MAX of the high speed component.

[0095] In a step S73, the computed deviation DHO2 is converted to anair-fuel ratio equivalent value, and a target air-fuel ratio T-A/F ofthe engine 1 is set.

[0096] Therefore, according to this routine, when the high speedcomponent HO2 of the oxygen storage amount does not reach a targetamount, the target air fuel ratio of the engine 1 is set to lean, andthe oxygen storage amount (high speed component HO2) is increased. Onthe other hand, when the high speed component HO2 exceeds the targetamount, the target air fuel ratio of the engine 1 is set to rich, andthe oxygen storage amount (high speed component HO2) is decreased.

[0097] Next, the overall action performed by the above control will bedescribed.

[0098] In the exhaust purification device according to this invention,when the engine 1 starts, computation of the oxygen storage amount ofthe catalyst 3 begins, and air fuel ratio control of the engine 1 isperformed so that the oxygen storage amount of the catalyst 3 isconstant to maintain the conversion efficiency of the catalyst 3 at amaximum.

[0099] The oxygen storage amount of the catalyst 3 is estimated based onthe air-fuel ratio of the exhaust gas flowing into the catalyst 3 andthe intake air amount, and computation of the oxygen storage amount isdivided into the high speed component HO2 and low speed component LO2according to the actual characteristics.

[0100] Specifically, the computation is performed assuming that whenoxygen is stored, the high speed component HO2 is preferentially stored,and the low speed component LO2 begins to be stored when the high speedcomponent HO2 can no longer be stored. The computation also assumes thatwhen oxygen is released, when the ratio (LO2/HO2) of the low speedcomponent LO2 and high speed component HO2 is less than thepredetermined value AR, oxygen is preferentially released from the highspeed component HO2, and when the ratio LO2/HO2 reaches thepredetermined value AR, oxygen is released from both the low speedcomponent LO2 and high speed component HO2 to maintain this ratioLO2/HO2.

[0101] When the high speed component HO2 of the computed oxygen storageamount is larger than the target value, the controller 6 decreases thehigh speed component by controlling the air-fuel ratio of the engine 1to rich, and when it is less than the target value, the high speedcomponent HO2 is increased by controlling the air-fuel ratio to lean.

[0102] As a result, the high speed component HO2 of the oxygen storageamount is maintained at the target value, and even if the air-fuel ratioof the exhaust flowing into the catalyst 3 shifts from thestoichiometric air-fuel ratio, oxygen is immediately stored as the highspeed component HO2 or immediately released as the high speed componentHO2 which has a high responsiveness, the catalyst atmosphere iscorrected to the stoichiometric air-fuel ratio, and the conversionefficiency of the catalyst 3 is maintained at a maximum.

[0103] Further, if computational errors accumulate, the computed oxygenstorage amount shifts from the real oxygen storage amount, however theoxygen storage amount (high speed component HO2 and low speed componentLO2) is reset with a timing at which the exhaust downstream of thecatalyst 3 becomes rich or lean, and any discrepancy between thecomputed value and real oxygen storage amount is corrected.

[0104]FIG. 11 shows how the high speed component HO2 varies when theabove oxygen storage amount constant control is performed.

[0105] In this case, at the time t1, the output of the rear oxygensensor 5 becomes less than the lean determining threshold and lean resetconditions hold, so the high speed component HO2 is reset to the maximumcapacity HO2MAX. However, the low speed component LO2 is not necessarilya maximum at this time, so reset of the low speed component is notperformed, not shown.

[0106] At times t2, t3, the output of the rear oxygen sensor 5 becomesgreater than the rich determining threshold and rich reset conditionshold, so the high speed component HO2 of the oxygen storage amount isreset to the minimum capacity (=0). The low speed component LO2 at thistime is also reset to the minimum capacity, not shown.

[0107] Thus, resetting of the computed values of the oxygen storageamount is performed with a timing at which the air-fuel ratio of theexhaust downstream of the catalyst 3 becomes rich or lean, and as aresult of the discrepancy from the real oxygen storage amount beingcorrected, the computational precision of the oxygen storage amount ofthe catalyst is further enhanced, the precision of air-fuel ratiocontrol for maintaining the oxygen storage amount constant is increased,and the conversion efficiency of the catalyst is maintained at a highlevel.

[0108] The above shows an example of an exhaust purification deviceassumed by this invention. In this invention, in such an exhaustpurification device which controls the oxygen storage amount of acatalyst to be constant, the object is to determine catalystdeterioration with high accuracy. Hereafter, this will be describedreferring to FIG. 12 and subsequent drawings.

[0109]FIG. 12 is a processing routine in a first embodiment fordetermining catalyst deterioration, and is periodically performed insynchronism with the air-fuel ratio control processing of FIG. 10. FIG.13 is a diagram showing how the oxygen storage amount varies when theaforesaid processing routines are executed. According to thisembodiment, the high speed component of the oxygen storage amount isbasically sampled a predetermined number of times, and an average valueof the oxygen storage amount computed from this integrated value iscompared with a predetermined reference value to determine catalystdeterioration.

[0110] In this processing, a deterioration determining permissioncondition is first determined in a step S81. This is done for example bydetermining whether or not a catalyst 3 is in an activated state basedon a water temperature or catalyst temperature, and permitting adeterioration determination to be made when the catalyst is in theactivated state. An integrated value SUMHO2 of the oxygen storage amountand a counter Csum for integral management are respectively initializedto 0, and the routine proceeds to a subsequent determination of adeterioration determining region condition (steps S82, S83). Thedeterioration determining region condition may for example be an enginerotation speed, fuel injection amount, vehicle speed or air-fuel ratiocontrol state, and a determination is performed as to whether or notrunning conditions determined from these are within predeterminedconditions. In this way, an appropriate deterioration determination canbe performed excluding running conditions which are unsuitable for thedetermination such as fuel cut during deceleration. When thedeterioration determining permission condition is not satisfied, thepresent processing is terminated, and the system enters a waiting stateuntil the condition is satisfied.

[0111] When catalyst deterioration is determined, it is first determinedwhether the oxygen storage amount is being controlled after theaforesaid rich reset, or controlled during the aforesaid lean reset in astep S84. This is determined for example by referring to flags Frich andFlean used in the processing of FIG. 9. Specifically, when the flagFrich=1 and the flag Flean=0, control is being performed after a richreset, and when Frich=0 and Flean=1, control is being performed afterthe lean reset. Herein, when control is being performed after the richreset, an oxygen storage amount HO2 is added to the integrated valueSUMHO2, and processing is performed to update SUMHO2 in a step S85. Onthe other hand, when control is being performed after a lean reset, theresult of subtracting the oxygen storage amount HO2 from the maximumoxygen amount HO2MAX of the catalyst is added to the integrated valueSUMHO2 to update SUMHO2 in a step S86. In FIG. 13, the symbol R denotesthe rich reset, and the symbol L denotes the lean reset. In thisexample, the deterioration determination starts immediately prior to therich reset.

[0112] The computation of the aforesaid integrated value SUMHO2 isrepeated until the counter value Csum reaches a predetermined samplingnumber Nc in steps S87, S88. In other words, due to this processing, theoxygen storage amount HO2 per unit time is integrated Nc times.

[0113] Next, the integrated value SUMU02 found in this way is divided bythe sampling number Nc to compute an average value AVHO2 of the oxygenstorage amount HO2 in a step S89, and this average value AVHO2 iscompared with a predetermined determining value. If AVHO2>thedetermining value, it is determined that the deterioration level isstill acceptable and the present processing is terminated, whereas ifAVHO2<the determining value, it is determined that the catalyst hasdeteriorated in steps S810, S811. The result of this deteriorationdetermination is stored for example in a self-diagnostic device of thevehicle, or it may be notified to the driver in real time by a monitorlamp or the like. FIG. 13 shows the result where AVHO2 is larger thanthe determining value, i.e., when deterioration of the catalyst 4 isstill OK.

[0114] In this embodiment, the oxygen storage amount of the catalyst maybe divided into a high speed component which is stored or released by acatalyst noble metal such as Pt, Rh, Pd, and a low speed component whichis stored or released relatively slowly by an oxygen storage materialsuch as ceria. As the high speed component has a high storage rate orrelease rate (referred to hereafter as “storage/release rate”) from thecatalyst compared to the low speed component, the oxygen storage amountis very sensitive to air-fuel ratio fluctuation and catalystdeterioration. Therefore, catalyst deterioration may be determined witha good response by using the result of integrating the high speedcomponent of the oxygen storage amount.

[0115] As one method of determining deterioration from the integratedvalue of the oxygen storage amount, a determining reference value may beprovided and determined relative to the integrated value, an averagevalue of the oxygen storage amount computed from this integrated value,and compared with the determining value. This allows the deteriorationto be determined with higher reliability.

[0116] There is no need to process detection parameters to determinedeterioration apart from those used in the control of the air-fuel ratioand oxygen storage amount, such as the computation result of the oxygenstorage amount, so the processing programme for determiningdeterioration can be simplified.

[0117] Further, the total oxygen storage amount is computed from thehigh speed component which has a relatively high oxygen storage/releaserate, and low speed component which has a lower oxygen storage/releaserate, so deterioration can be determined with still higher accuracy.

[0118]FIG. 14 is showing a processing routine in a second embodiment fordetermining catalyst deterioration, and is periodically performed insynchronism with the air-fuel ratio control processing of FIG. 10. FIG.15 is a diagram showing how the oxygen storage amount varies when theaforesaid processing routine is executed. According to this embodiment,the catalyst deterioration is determined by sampling the high speedcomponent of the oxygen storage amount a predetermined number of timeseach time the reset processing is performed, and comparing the averagevalue of the oxygen storage amount computed from this integrated valuewith a determining value.

[0119] In this processing, a deterioration determining permissioncondition is determined in the same way as in FIG. 12 (a step S91), andwhen deterioration determination in the catalyst activation state ispermitted, the integrated value SUMHO2 of the oxygen storage amount anda counter Cres for integral management are respectively initialized to0, and the routine then shifts to determination of the deteriorationdetermining region condition in steps S92, S93. The deteriorationdetermining region condition is identical to that of FIG. 12, and isbased on various engine running state parameters including the enginerotation speed. When the deterioration determining permission conditionis not satisfied, the present routine is terminated, and the systemwaits until the condition is satisfied.

[0120] In determining catalyst deterioration, it is first determinedwhether or not a rich reset or lean reset is performed by referring tothe flags Frich and Flean used in the processing of the FIG. 9 (a stepS94). Here, it is known that when the flag Frich inverts from 0 to 1, arich reset is performed, and when the flag Flean inverts from 0 to 1, alean reset is performed. When a lean reset is performed, the oxygenstorage amount HO2 immediately prior to reset is added to the integratedvalue SUMHO2 to update SUMHO2 in a step S95. Conversely, when a richreset is performed, the result of subtracting the storage amount HO2immediately prior to reset from the maximum oxygen storage amount HO2MAXis added to the integrated value SUMHO2 to update SUMHO2 in a step S96.

[0121] The computation of the integrated value SUMHO2 is repeated untilthe counter value Cres reaches a predetermined number Nr in steps S97,S98. In other words, due to this processing, the oxygen storage amountHO2 immediately prior to reset during the interval when rich reset orlean reset is performed Nr times, is integrated.

[0122] Next, the integrated value SUMU02 found in this way is divided bythe detection number Nr to compute the average value AVHO2 of the oxygenstorage amount HO2 in a step S99, and this average value AVHO2 iscompared with a predetermining value. When AVHO2>the determining value,it is determined that the deterioration level is still acceptable andthe routine is terminated, whereas when AVHO2<the determining value, itis determined that the catalyst has deteriorated in steps S910, S911.The result of this determination may be stored for example in aself-diagnostic device of the vehicle, or may be notified to the driverin real time by a monitor lamp or the like. FIG. 15 shows the case wherethe average value AVHO2 is larger than the determining value, i.e. itshows the determining result when there is no deterioration.

[0123] As the above embodiment, when the deterioration is determined byintegrating the oxygen storage amount each time reset processing isperformed which initializes the oxygen storage amount, the oxygenstorage amount may for example be detected before each reset, and anintegrated value calculated by integrating this, excluding oxygenstorage amounts which had a large variation due to reset processing,hence the deterioration may be detected with higher accuracy. It may benoted that the integrated value of the oxygen storage amount may bedetermined during a predetermined time span or number of samplingsregardless of whether or not reset is performed, and in this case, asthe duration can be determined without waiting for reset processing, thedeterioration determining timing can be shortened.

[0124]FIG. 16 is showing a routine in a third embodiment for determiningcatalyst deterioration, and is periodically performed in synchronismwith the processing of the air-fuel ratio control of FIG. 10. In thisembodiment, the reset frequency of the high speed component is detected,and the catalyst deterioration is determined by comparing this with adetermining value.

[0125] In this processing, a deterioration determining permissioncondition is first determined as in FIG. 12 (step S101). Whendeterioration determination is permitted in the catalyst activationstate, an integrated value SUMTint between resets and a counter Cint forintegral management are respectively initialized to 0, and the routinethen shifts to determination of the following deterioration determiningregion condition in steps S102, S103. The deterioration determiningregion condition is identical to that of FIG. 12, and is determinedbased on various running parameters including the engine rotation speed.When the deterioration determining permission condition is notsatisfied, the present processing is terminated, and the system waitsuntil the condition is satisfied.

[0126] To determine the catalyst deterioration, the presence or absenceof rich reset or lean reset is first detected, and when either of theseis performed, a time Tint until the next reset is performed is measuredin a step S104. This measures the time from R to L in FIG. 13 or from Lto R in FIG. 15 by for example referring to the flags Frich and Fleanused in the processing of FIG. 9.

[0127] Next, the measured time Tint is added to an integrated valueSUMTint on each occasion that this time measurement is performed toupdate SUMTint, and increase the counter value Cint. This processing isrepeated until the counter value Cint reaches a predetermined number Ni,i.e., the reset time interval Tint is integrated Ni times in steps S105,S107.

[0128] Next, the integrated value SUMTint found in this way is dividedby the detection number Ni to compute an average value AVTINT of thereset time interval in a step S108, and this average value AVTINT iscompared with a predetermined determining value. When AVTINT>thedetermining value, it is determined that the deterioration level isstill acceptable and the present processing is terminated, and whenAVTINT<the determining value, it is determined that there is catalystdeterioration in steps S109, S110. The result of this deteriorationdetermination is for example stored in a self-diagnostic device of thevehicle, or catalyst deterioration is notified in real time to thedriver by a monitor lamp or the like based on this result.

[0129] In this embodiment, when catalyst deterioration progresses, theoxygen storage amount decreases, so the fluctuation width of the airfuelratio of the catalyst atmosphere increases during the air-fuel ratiocontrol process, and the frequency with which a lean determining valueor rich determining value is exceeded, i.e. the frequency of resetprocessing, increases. Therefore, the frequency of this reset processingmay be monitored, and the catalyst determined to have deteriorated whenthis exceeds a predetermined reference value.

[0130] There is no need to process detection parameters to determinedeterioration apart from those used in the control of the air-fuel ratioand oxygen storage amount, such as the computation result of the oxygenstorage amount, so the processing programme for determiningdeterioration can be simplified

[0131] Further, the total oxygen storage amount is computed from thehigh speed component which has a relatively high oxygen storage/releaserate, and low speed component which has a lower oxygen storage/releaserate, so deterioration can be determined with still higher accuracy.

[0132] The embodiments of this invention in which an exclusive propertyor privilege is claimed are defined.

[0133] The contents of Japanese Application No-2000-49185, with a filingdate Feb. 25, 2000 is hereby incorporated by reference.

[0134] The embodiments of this invention in which an exclusive propertyor privilege is claimed are defined.

[0135] Industrial Applicability

[0136] As described above, the exhaust purification device according tothe present invention is useful as an exhaust purification devicepermitting determination of catalyst deterioration without depending onthe output inversion of the exhaust oxygen sensor.

1. An engine exhaust purification device comprising: a catalyst (3)provided in an engine exhaust passage (2), a sensor (4) which detects anexhaust characteristic flowing into the catalyst (3), and amicroprocessor (6) programmed to compute an oxygen storage amount of thecatalyst (3) using the detected exhaust characteristic, compute a targetair-fuel ratio of the engine (1) based on the computed oxygen storageamount such that the oxygen storage amount of the catalyst is apredetermined target value, and determine a deterioration of thecatalyst (3) based on an integrated value of the oxygen storage amountfor a predetermined time.
 2. The engine exhaust purification device asdefined in claim 1, further comprising a second sensor (5) which detectsan air-fuel ratio of an exhaust flowing out of the catalyst (3), and themicroprocessor (6) is further programmed to: perform reset processingwhich initializes the oxygen storage amount to a maximum value when theair-fuel ratio of the exhaust from the catalyst (3) detected via thesecond sensor (5) exceeds a lean determining value, and initializes theoxygen storage amount to a minimum value when the air-fuel ratio of theexhaust from the catalyst (3) detected via the second sensor (5) exceedsa rich determining value, and determine that the catalyst (3) hasdeteriorated based on the integrated value of the oxygen storage amounton each occasion reset processing is performed.
 3. The engine exhaustpurification device as defined in claim 1, the oxygen storage amount ofthe catalyst (3) is a high speed component which has a relatively highoxygen absorption/release rate.
 4. The engine exhaust purificationdevice as defined in claim 1, wherein the microprocessor (6) is furtherprogrammed to compare the integrated value of the oxygen storage amountwith its average value, and determine that the catalyst (3) hasdeteriorated when the integrated value is equal to or less than thedetermining value.
 5. An engine exhaust purification device comprising:a catalyst (3) provided in an engine exhaust passage (2), a first sensor(4) which detects an exhaust characteristic of flowing into the catalyst(3), a second sensor (5) which detects an air-fuel ratio of exhaustflowing out of the catalyst (3), and a microprocessor (6) programmed to:compute the oxygen storage amount of the catalyst (3) using the detectedexhaust characteristic, perform reset processing which initializes theoxygen storage amount to a maximum value when the air-fuel ratio of theexhaust from the catalyst (3) detected via the second sensor (5) exceedsa lean determining value, and initializes the oxygen storage amount to aminimum value when the air-fuel ratio of the exhaust from the catalyst(3) detected via the second sensor (5) exceeds a rich determining value,compute a target air-fuel ratio of an engine (1) so that the oxygenstorage amount of the catalyst (3) is a predetermined target value basedon the computed oxygen storage amount, and compare a reset processingfrequency with a determining value, and determine that the catalyst (3)has deteriorated when the reset processing frequency exceeds thedetermining value.
 6. The engine exhaust purification device as definedin the one of claims 1 to 5, wherein the microprocessor (6) isprogrammed to compute the catalyst oxygen storage amount from a highspeed component which has a relatively high oxygen absorption/releaserate, and a low speed component which has a lower oxygenabsorption/release rate than the high speed component.
 7. The engineexhaust purification device as defined in the one of claims 1 or 5,wherein the exhaust characteristic is air-fuel ratio or an oxygenconcentration.
 8. An engine exhaust purification device comprising: acatalyst (3) provided in an engine exhaust passage (2), means (4) fordetecting an exhaust characteristic flowing into the catalyst (3), meansfor computing an oxygen storage amount of the catalyst (3) using thedetected exhaust characteristic, means for computing a target air-fuelratio of the engine (1) based on the computed oxygen storage amount suchthat the oxygen storage amount of the catalyst is a predetermined targetvalue, and means for determining a deterioration of the catalyst (3)based on an integrated value of the oxygen storage amount for apredetermined time.
 9. A method for determining a deterioration of thecatalyst, comprising: computing an oxygen storage amount of the catalyst(3) using the detected exhaust characteristic, computing a targetair-fuel ratio of the engine (1) based on the computed oxygen storageamount such that the oxygen storage amount of the catalyst is apredetermined target value, and determining a deterioration of thecatalyst (3) based on an integrated value of the oxygen storage amountfor a predetermined time.
 10. An engine exhaust purification devicecomprising: a catalyst (3) provided in an engine exhaust passage (2),means (4) for detecting an exhaust characteristic of flowing into thecatalyst (3), means (5) for detecting an air-fuel ratio of exhaustflowing out of the catalyst (3), means (6) for computing the oxygenstorage amount of the catalyst (3) using the detected exhaustcharacteristic, means (6) for performing reset processing whichinitializes the oxygen storage amount to a maximum value when theair-fuel ratio of the exhaust from the catalyst (3) detected via thesecond sensor (5) exceeds a lean determining value, and initializes theoxygen storage amount to a minimum value when the air-fuel ratio of theexhaust from the catalyst (3) detected via the second sensor (5) exceedsa rich determining value, means (6) for computing a target air-fuelratio of an engine (1) so that the oxygen storage amount of the catalyst(3) is a predetermined target value based on the computed oxygen storageamount, and means (6) for comparing a reset processing frequency with adetermining value, and means (6) for determining that the catalyst (3)has deteriorated when the reset processing frequency exceeds thedetermining value.
 11. A method for determining a deterioration of thecatalyst, comprising: computing the oxygen storage amount of thecatalyst (3) using the detected exhaust characteristic, performing resetprocessing which initializes the oxygen storage amount to a maximumvalue when the air-fuel ratio of the exhaust from the catalyst (3)detected via the second sensor (5) exceeds a lean determining value, andinitializes the oxygen storage amount to a minimum value when theair-fuel ratio of the exhaust from the catalyst (3) detected via thesecond sensor (5) exceeds a rich determining value, computing a targetair-fuel ratio of an engine (1) so that the oxygen storage amount of thecatalyst (3) is a predetermined target value based on the computedoxygen storage amount, and comparing a reset processing frequency with adetermining value, and determining that the catalyst (3) hasdeteriorated when the reset processing frequency exceeds the determiningvalue.